A fuel cell is an electrochemical system in which a fuel (such as hydrogen) is reacted with an oxidant (such as oxygen) at high temperature to generate electricity. One type of fuel cell is the solid oxide fuel cell (SOFC). The basic components of a SOFC may include an anode, a cathode, a solid electrolyte, and an interconnect. The fuel may be supplied to the anode, and the oxidant may be supplied to the cathode of the fuel cell. At the cathode, electrons ionize the oxidant. The electrolyte comprises a material that allows the ionized oxidant to pass through to the anode while simultaneously being impervious to the fluid fuel and oxidant. At the anode, the fuel is combined with the ionized oxidant and releases electrons to be conducted back through an external circuit to the cathode. Additional heat, generated in the stack from ohmic losses, is transferred to the cathode stream. This heat can either be used to facilitate other chemical reactions within the system, it can be exhausted from the system, or is radiated to the environment.
A SOFC may be structured, e.g., as a segment-in-series or in-plane series arrangement of individual cells. The oxidant is typically introduced at one end of the series of cells and flows over the remaining cells until reaching the cathode exhaust outlet. Each fuel cell transfers a portion of the ohmic heat into the oxidant thereby raising its temperature, and forming a temperature gradient which increases from the oxidant inlet to the exhaust. Consequently, a temperature gradient may also develop in the fuel cell which increases from the oxidant inlet to the oxidant exhaust. This temperature gradient may cause thermal stresses leading to degradation or failure of the fuel cell components.
The anode of a SOFC may be a mixed cermet comprising nickel and zirconia (such as, e.g., yttria stabilized zirconia (YSZ)) or nickel and ceria (such as, e.g., gadolinia doped ceria (GDC)). Nickel, and other materials, function not only to support the chemical reaction between the fuel and the ionized oxidant but also have catalytic properties which allow the anode to reform a hydrocarbon fuel within the fuel cell. One method of reforming the hydrocarbon fuel is steam reforming of methane (CH4) to form syngas, an endothermic reaction:CH4+H2O→CO+3H2ΔH°=206.2 kJ/mole
The heat necessary for the reformation of methane could be supplied directly from the ohmic heat generated within the fuel cell stack. This direct heat transfer would help cool the stack, reduce thermal stresses, and enable more of the fuel cell stack to operate at the optimal operating temperature for the fuel cells. However, in-stack reforming introduces several technical challenges. Unreformed methane must be supplied in the correct amount to avoid excessively cooling of the fuel cell and in the correct manner to avoid localized cooling. Additionally, under the right conditions, hydrocarbon fuels have a propensity to form carbon, for example via thermal cracking:CxH2x+2→C+(x+1)H2 Carbon formation can cause fouling and degradation of fuel cell components through anode delamination, metal dusting and other failure mechanisms.
Consequently, supplying a mixture of a syngas reformed external to the fuel cell and an unreformed fuel to the anode may provide better a balance of system performance and durability than supplying either reformate or unreformed fuel alone. However, the ratio of reformed and unreformed fuel must be precisely controlled. If the ratio is too high, the large temperature gradient across the fuel stack will remain. If it is too low, carbon formation will compromise fuel cell performance and life.
Additionally, assemblies for controlling the flow rate a fluid typically include needle or other types of valves and orifice plates. Some adjustable orifice plates comprise rotating plates wherein each plate defines an opening. The alignment of plate openings determines the effective flow area of the orifice. However, these solutions are not suitable for the high temperature and pressure conditions of an operating fuel cell and are prone to leakage.
There remains a need for precise control of the ratio of reformed and unreformed fuels delivered to a fuel cell stack to ensure that the proper amount of reforming occurs internally to the fuel cell. Additionally, there remains a need for systems and methods to achieve this precise control.
In accordance with some embodiments of the present disclosure, a reformer with a bypass is provided. The bypass may contain a flow controller that restricts the bypass flow. The flow controller may be adjustable to control the flow rate of the fluid through the bypass, thereby enabling precise control of the ratio of reformate and unreformed fuels supplied to the fuel cell stack. This design permits some of the disclosed embodiments to accommodate a wide range of in-stack reforming fuel cell designs and minimizes the risk for carbon formation.
In accordance with some embodiments of the present disclosure, effective and adjustable means are provided that control the fluid flow rate within a reformer bypass in a high temperature and pressure environment.
In accordance with some embodiments of the present disclosure, a reformer unit is provided. The reformer unit may have a reforming section, a heat exchanging section, and a bypass section. The reforming section may reform a hydrocarbon-containing fuel, and have an inlet in fluid communication with a source of hydrocarbon fuel and an outlet in fluid communication with an anode inlet of a fuel cell stack. The heat exchanging section may heat a fluid flowing in the reforming section, in the bypass section, or both, and may have an inlet in fluid communication with an exhaust of a cathode of a fuel cell stack, and an outlet adapted for fluid communication with an inlet of a cathode of a fuel cell stack. The heat exchanging section is in thermal communication with said reforming section (or said bypass section or both) to effect heat transfer between the fluids flowing in each section. The bypass section provides a flow path for the hydrocarbon-containing fuel around the reforming section, and has an inlet in fluid communication with the reforming section inlet, an outlet in fluid communication with the reforming section outlet, and a variable orifice flow controller positioned in the bypassing flow path.
In accordance with some embodiments of the present disclosure, a variable orifice flow controller for controlling the flow of a high temperature, high pressure, or both fluid is provided. The flow controller may comprise an upstream connector, a downstream connector and an interconnector. The upstream connector may have cylindrical tubular portion defining a conduit in fluid communication with a flow path of high temperature fluid and a frusto-conical portion defining a plurality of conduits in fluid communication with the conduit. The downstream connector may define a frusto-conical cavity for receiving the frusto-conical portion of said upstream connector and a plurality of conduits in fluid communication with said cavity. The interconnector may provide a fluid-tight connection when the frusto-conical portion of the upstream connector is received within the cavity defined by said downstream connector. The amount of fluid communication between the plurality of conduits defined by the downstream and upstream connectors is selected by the radial alignment between the upstream and downstream connectors when in a gastight connection.
In accordance with some embodiments of the present disclosure a variable orifice flow controller for controlling the flow of a high temperature, high pressure, or both gas is provided. The flow controller may comprise an upstream connector, a downstream connector, a disc, and an interconnector. The upstream connector may have cylindrical tubular portion defining a conduit in fluid communication with a flow path of high temperature fluid and a frusto-conical portion defining a plurality of conduits in fluid communication with the conduit. The downstream connector may define a frusto-conical cavity for receiving the frusto-conical portion of said upstream connector and a conduit in fluid communication with the cavity. The disc may define a plurality of conduits and may be adjacent to a face of the frusto-conical portion of said upstream connector in a selected radial alignment such that the plurality of conduits defined by the disc are in fluid communication with the plurality of conduits defined by the frusto-conical portion of the upstream connector and the conduit define by the downstream connector. The interconnector may provide a fluid-tight connection when the frusto-conical portion of the upstream connector is received within the cavity defined by said downstream connector. The amount of fluid communication between the plurality of conduits define by the disc and the plurality of conduits defined by the frusto-conical portion of said upstream connector is selected by the radial alignment of the conduits when in a gastight connection.
In accordance with some embodiments of the present disclosure, a reformer unit for a fuel cell is presented. The reformer unit may comprise a reforming section, a heat exchanging section, and a bypass plenum. The reforming section reforms a hydrocarbon-containing fuel and has an inlet in fluid communication with a source of hydrocarbon-containing fuel and an outlet plenum in fluid communication with an anode inlet of a fuel cell stack. The heat exchanging section heats a fluid flowing in the reforming section, the bypass plenum, or both. The heat exchanging section has an inlet in fluid communication with the exhaust of a cathode and an outlet adapted for fluid communication with an inlet of cathode of the fuel cell stack. The heat exchanging section is in thermal communication with the reforming section and the bypass plenum to effect a heat transfer. The bypass plenum provides a flow path for the hydrocarbon-containing fuel to bypass the reforming section and has an inlet in fluid communication with the reforming section inlet, an outlet in fluid communication with the reforming section outlet plenum and a flow restrictor in the flowpath between the outlet of the bypass plenum and the outlet plenum of the reforming section.
In accordance with some embodiments of the present disclosure, a flow restrictor for restricting the flow of a high temperature fluid through an orifice providing fluid communication between two plenums is provided. The flow restrictor may comprise a connector mounted to a wall of a first plenum, a fitting, an elongated flow restricting member, and an internally threaded sealing nut. The connector comprises a first portion defining a cylindrical cavity having a threaded portion and a second portion which defines a frusto-cylindrical cavity in communication with the cylindrical cavity. The fitting comprises a frusto-conical end portion that is positioned within the frusto-conical cavity and defines an axial slot. The elongated flow restricting member comprises a cylindrical threaded portion positioned and threadably engaged with the cylindrical cavity, a portion extending from one end of said cylindrical portion into the axial slot and a tapered portion extending from the other end of the cylindrical portion through the orifice. The axial alignment of the tapered portion and the orifice is selectable by rotating the flow restricting member relative to the connector. The internally threaded sealing nut engages an external threaded portion of the connector and provides a fluid-tight seal between the fitting and the connector.
In accordance with some embodiments of the present disclosure, a reforming unit for a fuel cell system is provided. The reforming unit may comprise a reforming section, a heat exchanging section and a bypass plenum. The reforming section reforms a hydrocarbon containing fuel. The heat exchanging section effects a heat transfer between a fluid flowing therethrough and the fluid flowing through the reforming section, the bypass plenum, or both. The bypass plenum provides a flowpath for the hydrocarbon-containing fuel to bypass the reforming section. The bypass plenum may comprise a flow restrictor in the outlet of the bypass plenum to control the amount of fluid communication between the outlet of the bypass plenum and the outlet of the reforming section.
These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detail description of the embodiments.
Referring to the drawings, some aspects of non-limiting examples of a fuel cell system in accordance with an embodiment of the present disclosure are schematically depicted. In the drawings, various features, components and interrelationships therebetween of aspects of an embodiment of the present disclosure are depicted. However, the present disclosure is not limited to the particular embodiments presented and the components, features and interrelationships therebetween as are illustrated in the drawings and described herein.